Image‐based method to measure and characterize shim‐induced eddy current fields

Abstract

Dynamic magnetic field shimming is gaining interest for field sensitive MRI acquisitions. Using slice based or real-time shim updating, significant improvements in static field (B 0 ) uniformity can be obtained. While the ability to rapidly switch shim fields can improve overall B 0 homogeneity, it induces eddy current fields that must be characterized and compensated for. Methods used to achieve this have thus far been based on linear projection spin echo sequences or field probe assemblies. Here, a novel imagebased method is presented to measure and characterize eddy current fields without the need for field probes or projection based measurements. This technique can be extended to characterize very high order spherical harmonic fields, making it a useful tool to calibrate next-generation shim systems implementing dynamic field steering with greater than third order shim terms. Results are used to calibrate a Dynamic Shim Updating unit for pre-emphasis and eddy current compensation. Three-dimensional datasets are acquired at multiple MR facilities containing complete spatiotemporal field information to compensate eddy current field self-and cross-terms for up to third order. Furthermore, simulation studies are performed to investigate the effect of scan resolution and phantom size with respect to accurate eddy current field characterization.

Fast iterative pre‐emphasis calibration method enabling third‐order dynamic shim updated fMRI

“…If this settling period is chosen too short, residual eddy currents may be present when the shim is switched off before the characterization measurement. Those eddy currents will be compensated by the switching off process and cannot be characterized (9,16). In contrast, FIG.…”

“…If this settling period is chosen too short, residual eddy currents may be present when the shim is switched off before the characterization measurement. Those eddy currents will be compensated by the switching off process and cannot be characterized . In contrast, spatiotemporal field monitoring allows the observation of the switching off, as well as the switching on process, and hence allows to determine if all eddy currents have settled before the current of the respective term is switched off again.…”

Interslice current change constrained B0 shim optimization for accurate high‐order dynamic shim updating with strongly reduced eddy currents

“…Therefore, sometimes up to 50% of the dynamic range of the power supply cannot be used for the purpose of shimming, and DSU optimization in its standard form can run into current limitations. Given that they require a full 4D characterization of the ECs generated by each shim coil, accurate pre‐emphasis adjustments are time‐consuming. Conventional MR sequence‐based EC measurement techniques, for example, based on multiple projections or full 4D EC maps, can take up to a 1‐hour measurement time per shim channel. Slight calibration inaccuracies can already translate into long‐lived and uncompensated ECs with amplitudes that scale with the initial shim current change. The shim basis functions are not necessarily orthogonal over the DSU volumes.…”

B0 magnetic field homogeneity and shimming for in vivo magnetic resonance spectroscopy

“…The rapid field switching with coils fitted to the scanner bore induces eddy currents in the magnet’s cold conducting structures that result in a multitude of artificial field terms throughout the bore. The quantitative spatial and temporal characterization of all of these field terms is necessary for the identification of significant components (37,49,50). Their experimental minimization by shim pre-emphasis and B 0 compensation is essential for successful higher SH order dynamic shimming (37,45,49).…”